This invention relates generally to improvements in methods and apparatus incorporating shape memory alloys, and, more particularly, to the use of nickel-titanium (NiTi) alloys as a medical device or a medical device component deformed in an austenitic state prior to being packed into a tube, catheter or the like, and subsequently deployed into a mammalian body for appropriate medical treatment.
It is well known in the field that both organic and metallic materials are capable of exhibiting shape memory effect. The nickel-titanium alloy known as “nitinol,” is an extensively used metal alloy for medical device applications and possesses unique properties under certain conditions. One benefit of applying nitinol to medical devices is that the alloy has tremendous elasticity and useful shape memory characteristics. In terms of its elasticity, nitinol can become highly elastic under certain conditions in that it is able to experience extensive deformation and yet transform back to its original shape. This beneficial attribute of nitinol is known as superelasticity, also commonly referred to as pseudoelasticity. Superelasticity or pseudoelasticity refers to the ability of a material to undergo extremely large elastic deformation. The shape memory properties of nitinol enable it to “remember” a particular shape instilled during a previous heat set operation and to transform back to that shape when desired. Furthermore, nitinol is highly biocompatible, kink resistant, fatigue resistant, and has other beneficial engineering attributes thus making the material particularly useful in medical applications.
In one particular application, nitinol has found use in self-expanding stents. Historically, stents were not self-expanding but rather were expanded and deployed by a balloon catheter. Balloon expanded stents are used in conjunction with balloon angioplasty procedures with the intent to reduce the likelihood of restenosis of a diseased vessel. Stents are also used to support a body lumen, tack-up a flap or dissection in a vessel, or in general where the lumen is weak to add support.
For balloon expandable stents, the stent is positioned over the balloon portion of a catheter and is expanded from a reduced delivery diameter to an enlarged deployment diameter greater than or equal to the inner diameter of the arterial wall by inflating the balloon. Stents of this type are expanded to an enlarged diameter through deformation of the stent, which then engages the vessel wall. Eventual endothelial growth of the vessel wall covers the stent.
Nitinol then found use in self-expanding stents, where deployment was a result of either shape-memory effect or superelasticity in the material rather than by use of a dilatation balloon. The stent once released from its delivery system assumed a pre-set shape in the body lumen. Such self-expanding stents are used to scaffold the inside circumference of a tubular passage such as an esophagus, bile duct, or blood vessel.
The benefits of using a superelastic nitinol material for self-expanding stents are primarily related to its large recoverable strain. The biocompatibility of nickel-titanium is also an attractive benefit for use of this material in stenting applications, because the stent remains in the patient as part of the treatment.
The use of nickel-titanium as a balloon-expandable stent is less common. The balloon-expandable and scaffolding capabilities of such stents are accomplished by setting the austenite finish temperature (Af) at about 55 degrees C. or well above body temperature. The stent is therefore completely martensitic before, during, and after balloon deployment. A significant disadvantage of such a balloon-expandable nitinol stent in its martensitic phase is that martensite is very soft. Therefore, the scaffolding function and hoop strength of the stent are diminished.
As briefly described above, superelasticity or pseudoelasticity, refers to the highly exaggerated elasticity or spring-back observed in many nickel-titanium alloys deformed above their austenite finish temperature (Af) and below the martensite deformation temperature (Md). Hence, nickel-titanium alloys can deliver over fifteen times the elasticity of a spring steel. The martensite deformation temperature (Md) is defined as the temperature above which martensite cannot be stress-induced. Consequently, nickel-titanium remains in its austenitic phase throughout an entire deformation test above Md.
The shape memory effect characteristic of nitinol is a result of metallurgical phase transformations. Depending on its temperature, the structural properties of nitinol enable it to function in two different states. At the lower temperature range, below a specified transition temperature, the nitinol alloy becomes more flexible and malleable and is said to be in the martensitic state. However, when heated above the specified transition temperature, the nitinol alloy transforms into its predetermined heat set shape in the austenitic state.
The alloy can also undergo a thermoelastic martensitic transformation from an austenitic state to a martensitic state without a change in temperature. This transformation involves two characters, Ms (the temperature at which the transformation begins) and Mf (the temperature at which the transformation finishes). Thus, when a shape memory alloy is at a temperature above Ms (when the austenite state is initially stable), but below Md (the maximum temperature at which martensite transformation can occur), and is stressed, it begins to elastically deform, and thereafter, at a critical stress, the alloy transforms from austenite to stress-induced martensite. In the case of austenitic transformation, the alloy in the martensitic state undergoes a reverse transformation when it is warmed to a temperature at which the alloy begins to transform back to austenite, referenced by the character, As, with Af as the temperature at which the reversion is complete. The martensite is stable at a temperature below As, and unstable at a temperature above As in which the martensite transforms back to austenite and the material assumes its original shape. This shape memory effect is characteristic of nearly all alloys that exhibit thermoelastic martensitic transformation. It is important to recognize, however, that the stress and strain ranges for the effect, in addition to the range of temperatures over which stress-induced martensite occurs, vary significantly depending on the type of alloy used. Various medical devices incorporating the use of shape memory elements have been known for a number of years.
The various medical devices adopting the use of shape memory elements into their design as referenced above essentially rely on the unique structural properties of shape memory alloys in order to achieve their desired effects. In other words, shape memory alloys retain their new shape when cooled to the martensitic state and thereafter deformed; however, these same shape memory alloys will recover their original shape when warmed to the austenitic state.
Notwithstanding the beneficial effects of shape memory alloys employed in a wide variety of medical applications, disadvantages are also apparent. First, due to the fact that shape memory alloys are typically very composition-sensitive, it can be quite difficult to accurately control their transformation temperatures. Second, as a shape memory alloy undergoes a transformation between the austenitic and martensitic states, there can be a significant lag such that a reversal of the state of a shape memory alloy may result in a temperature set-off of several tens of degrees Celsius. In addition, the use of shape memory alloys in medical devices is expected to be limited due to the difficulties associated with temperature control and the biological limitations of human tissue in its ability to be repeatedly heated and cooled within certain narrow restraints (approximately 0°–60° C. for short periods of time) so as not to sustain any temporary or permanent damage.
What has been needed and heretofore not been used in the engineering of medical devices incorporating shape memory alloy elements is a method and apparatus for avoiding stress-induced martensitic transformation in nickel titanium alloys so that these type of alloys may instead be deformed in the austenitic state prior to their packing within a catheter, tube or the like, and hence avoid the extreme low temperatures normally used to deform such alloys. The present invention satisfies this need and others.